Metal substrate with enhanced corrosion resistance and improved paint adhesion

ABSTRACT

This invention is a blended powder mixture including a thermosetting resin and a solid, non-hydrolyzed, functional low molecular weight organosilane coupling agent. The powder mixture may be used to form a composite coating to protect a steel substrate from corrosion. The composite coating is formed by coating the substrate with an inorganic layer such as a silicate, depositing the powder mixture over the inorganic layer, heating the substrate to a temperature at least equal to the curing temperature of the resin and maintaining the substrate at the curing temperature for sufficient time to crosslink the resin and to diffuse the organosilane to the substrate whereby the crosslinked layer is coupled to the inorganic layer by the organosilane. The powder mixture includes 0.01-10 wt. % organosilane having a melting point no greater than the curing temperature of the resin. Preferred thermosetting resins include epoxy, polyester, epoxy-polyester, acrylic, acrylic-urethane or fluorovinyl.

This application is a division of U.S. Ser. No. 08/259,754; filed Jun.10, 1994; now U.S. Pat. No. 5,455,080 and a continuation of U.S. Ser.No. 07/936,197; filed Aug. 26, 1992, now abandoned; both applicationsincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to an organic powder composition and a method ofusing the powder composition for forming a crosslinked paint coupled toa metal substrate for protecting the substrate from corrosion. Moreparticularly, the powder composition includes a homogeneous mixture of athermosetting resin and a solid, non-hydrolyzed, functional organosilanecoupling agent.

It is known to form a composite coating on steel sheet for providingcorrosion protection. U.S. Pat. No. 5,108,793; incorporated herein byreference, discloses rinsing a steel sheet with an alkaline solutioncontaining a silicate, drying the sheet to form an insoluble inorganicsilicate coating, rinsing the coated sheet in an aqueous solutioncontaining an organosilane coupling agent and then drying the sheetforming a silane film on the outer surface of the silicate coating. Thesilicate coating provides excellent corrosion protection of the steelsheet. If the sheet is painted, the silane film acts as a coupling agentforming a strong covalent bond between the silicate coating and thepaint. An embodiment of this patent includes cold-rolled and galvanizedsteel sheets initially treated with a phosphate conversion coating, thentreated with the two step silicate/silane rinse coating and lastlyelectrostatically coated with a powder paint or cathodicallyelectrocoated with a paint for forming a composite coating. Thesilicate/silane layer improves corrosion protection and strengthens thebond between the inner phosphate layer and the outer paint layer. Adisadvantage with this process is that some of the silicate inevitablybecomes transferred to the silane rinse tank thereby contaminating thesilane solution. This contamination may reduce the effectiveness of thesilane as a coupling agent. Another disadvantage is that hydrolyzedsilane solutions have a relatively short storage life. Organosilanolsolutions tend to condense and polymerize thereby minimizing theirchemical interaction with paints.

It is well known to provide corrosion protection to steel sheet bycoating with a liquid resin containing an organosilane coupling agent.These liquid resins have the same disadvantage referred to above in thathydrolyzed silanes have reduced effectiveness as coupling agents.

More recently, it has become known to provide corrosion protection tosteel sheet by coating the sheet with a powder containing a siliconeresin. U.S. Pat. No. 5,077,354 discloses an in-situ formed polymericmixture comprising an acrylic graft copolymer of silicone resin used asa polymeric binder in powder paint. Silicone resin and ethylenicmonomers are dissolved in a solvent heated to 50°-150° C. and mixeduntil polymerization is completed. The solvent then is stripped awayleaving a non-gelled polymerized binder. A disadvantage of using thispowder paint is believed to be that a good bond is not formed betweenthe paint and steel substrate. Since the silicone resin has already beenreacted when in-situ combined with the ethylenic monomers when formingthe acrylic polymer, the silicone resin will not act as a coupling agentbetween the outer acrylic layer and the steel substrate.

Accordingly, there is a need to provide an organosilane having a longstorage life that can be used as a coupling agent when forming acrosslinked coating. There is a further need for developing a low cost,environmentally safe process for forming an adherent crosslinked coatingcoupled to a metal substrate such as cold-rolled steel or galvanizedsteel.

BRIEF SUMMARY OF THE INVENTION

The invention relates to an organic powder composition and a method ofusing the powder composition for forming a crosslinked paint coupled toa metal substrate for protecting the substrate from corrosion. Thepowder composition of the invention includes a homogeneous mixture of athermosetting resin and a solid, non-hydrolyzed, functional lowmolecular weight organosilane coupling agent. The organosilane is asolid at ambient temperature, has a melting point no greater than thecuring temperature of the resin and is readily dissolved into the resinduring crosslinking on the substrate.

Another feature of the invention includes forming a corrosion resistantcoating on a metal substrate using the aforesaid powder mixture with thecoating being coupled to the substrate by the organosilane. The coatingis the reaction product formed by in-situ curing of the powder mixtureafter being deposited onto the substrate. Preferably, the crosslinkedlayer has a thickness of at least 10 μm.

Another feature of the invention is for the aforesaid corrosionresistant coating to be a composite coating including a non-toxicinorganic inner layer coupled to the outer crosslinked layer by theorganosilane.

A further feature of the invention is forming a corrosion resistantcoating on a metal substrate including the steps of providing ahomogeneous powder mixture of a thermosetting resin and a solid,non-hydrolyzed, functional low molecular weight organosilane couplingagent, depositing a layer of the powder mixture onto the substrate,heating the substrate to a temperature at least equal to the curingtemperature of the resin and maintaining this temperature for sufficienttime to melt the resin and the organosilane. The resin is cured andforms a crosslinked layer that becomes tightly bonded to the substrateby the organosilane.

Another feature of the invention is for the aforesaid powder mixture tohave an organosilane concentration of 0.01-10 wt. %.

Another feature of the invention includes the additional step ofdepositing an inorganic layer onto the substrate prior to depositing thepowder layer.

Another feature of the invention is for the aforesaid crosslinked layerto have a thickness of at least 10 μm.

Another feature of the invention is for the aforesaid resin to have acuring temperature of 100°-250° C.

Another feature of the invention is for the aforesaid inorganic layer toinclude a silicate.

Another feature of the invention is for the aforesaid resin being fromthe group consisting of epoxy, polyester, epoxy-polyester, acrylic,acrylic-urethane or fluorovinyl.

An object of the invention is to form a corrosion resistant adherentcoating on a metal substrate using an organosilane coupling agent havinga long storage life.

Another object is to form a corrosion resistant adherent compositecoating on a metal substrate having an inorganic inner layer formed froma rinse solution having an extended storage life.

Another object is to provide a low cost, environmentally safe, processfor forming a corrosion resistant adherent coating on cold-rolled orgalvanized steel sheets using an organosilane coupling agent.

Advantages of the invention include a long storage life for anorganosilane coupling agent, forming a corrosion resistant coatinghaving good adherence to a variety of metal surfaces without using anenvironmentally hazardous waste substance, e.g. chromate rinses, and nocontamination of a rinse solution containing an inorganic coatingmaterial.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention includes providing a homogeneously blended powdercomposition, i.e., a paint, including a thermosetting resin and a solid,non-hydrolyzed, functional low molecular weight organosilane couplingagent that is soluble in the thermosetting resin. The organic resin maybe any thermosetting polymer such as epoxy, polyester, epoxy-polyester,acrylic, acrylic-urethane or fluorovinyl. The silane may be any organiccompound so long it is solid at ambient temperature, has a meltingtemperature no higher than the curing temperature of the resin, capableof being dissolved into the resin and does not include OH groups. By alow molecular weight silane is meant monomeric, dimeric or oligomericmolecules, i.e., RSi(OX)₃.

Hydrolyzed organosilanes, such as NH₂ -(CH₂)₃ Si(OH)₃, are undesirablebecause they are liquid at ambient temperature and tend to polymerizequickly. Although solid, non-hydrolyzed organosilanes react more slowlythan hydrolyzed organosilanes, the non-hydrolyzed forms do notpolymerize as rapidly. For epoxy, polyester and acrylic resins, thefollowing commercially available alkoxy silanes and chlorosilanes couldbe used: phenylallyldichlorosilane, neophylmethyldiethoxysilane (or thedimethoxy version), n-octadecyldimethylmethoxysilane,n-octadecyldimethyl[3-trimethoxysilyl-propyl] ammoniumchloride,2-(3,4-epoxyclyclohexyl)ethyltrimethoxysilane,t-butyldiphenylmethoxysilane, and styrylethyltrimethoxysilane. Forfluorovinyl resins such as PVDF or PTFE, the following commerciallyavailable silanes could be used:[2-(perfluorohexyl)ethyl]trichlorosilane,[2-(perfluorooctyl)ethyl]trichlorosilane, 1 H, 1 H, 2H, 2H-perfluoralkyltriethoxysilane, 3,3,3-trifluoropropyltriethoxysilane or(heptafluoroisopropoxy)propyitriethoxysilane. A preferred silane is3-(N-styrylmethyl-2-aminoethylamino)-propyltrimethoxysilanehydrochloride having the following structure:##STR1##

The blended resin/organosilane mixture preferably is used as paint withthe mixture including one or more additional powder additives such as apigment and a filler. For example, one or more of titanium oxide, leadoxide, zinc oxide, silica, calcium oxide, calcium carbonate, chromiumoxide, carbon black and mixtures thereof may be used as a pigment. Oneor more of clay, silica, talc, mica, woolastomite and wood flower may beused as a filler. The powders of the mixture preferably have a particlesize less than 325 mesh. The curing temperature of the resin preferablyis within the range of about 100°-250° C. The composition of themixture, including any additives, preferably should be 0.01-10 wt. %organosilane, more preferably at least about 0.2 wt. % organosilane. Theconcentration of the organosilane should be at least 0.01 wt. % becausethis amount is needed so that the silane diffuses to the interface ofthe metal substrate. The concentration of the organosilane should notexceed 10 wt. % because it may adversely affect the paint properties,may cause the paint to become plasticized and may weaken the covalentbond between the paint layer and the metal substrate by weak boundarylayer effects.

The blended resin/organosilane mixture will form a very adherent,corrosion resistant, crosslinked coating or paint coupled to a metalsubstrate by the silane. The coating can be formed on a variety of metalsurfaces including, but not limited to, hot rolled and pickled steelsheets, cold-rolled steel sheets, stainless steel sheets, hot dipmetallic coated steel sheets, electroplated metallic coated steelsheets, aluminum sheets and aluminum alloy sheets. The metallic coatingmay include one or more layers of lead, lead alloy, nickel, nickelalloy, nickel terne, zinc, zinc alloy, aluminum, aluminum alloy, and thelike. A phosphate conversion coating also may be applied to these steelsheets prior to being coated with the resin/organosilane powder mixture.The metal substrate may include continuous strip and foil, sheets cut tolength as well as bars, angles, tubes and beams.

After the resin/organosilane powder mixture is applied to a metalsubstrate, the substrate is heated to at least the curing temperature ofthe resin. The resin and organosilane melt with the silane beingdissolved into the liquid resin. Prior to the resin being crosslinkedand becoming solid, the silane will have interacted with the resin andhave migrated to the surface of the substrate. The silane reacts with OHgroups on the surface of the substrate forming a strong covalent bond ofthe type: ##STR2## where X is a metal oxide on the surface of thesubstrate or an inorganic layer. The oxides of some metals include OHgroups. In the case where the metal substrate is aluminum, an aluminumalloy or a steel sheet coated with aluminum or an aluminum alloy, thesilane may react with the OH groups present in the aluminum oxidesurface of the substrate forming a covalent bond of the typeMe--O--Si--. In the case where the metal substrate is steel or a steelsubstrate coated with a metallic coating other than aluminum or analuminum alloy, the substrate first should be preferably coated with aninner inorganic silicon containing layer having OH groups prior todeposition of the resin/organosilane powder mixture. The inorganic layermay be a silicate or a silica. In the latter case, the silane reactswith the OH groups present in the inorganic silicon containing layerforming a strong covalent bond of the type Si--O--Si-- which ispreferred to the Me--O--Si-- bond described above.

While the silane is reacting with the OH groups on the surface of themetal substrate during curing of the resin, the functional group of thesilane also will have interacted with a functional group in the resin orthe crosslinker in the resin, e.g., ##STR3## where R may be --(CH₂)₃Si(OCH₃)₃ and R' is the thermosetting resin, e.g., epoxy.

In a preferred embodiment, the corrosion resistant coating is acomposite of the previously described inner inorganic layer and theouter crosslinked paint layer. Preferably, the inner layer is a silicateformed in a manner as described in U.S. Pat. No. 5,108,793; incorporatedherein by reference. Waterglass is dissolved in water, e.g., an alkalinesolution containing Ba(NO₃)₃ addition. The solution preferably ismaintained at an elevated temperature, e.g., 55°-60° C., to reduce therinse time. A steel strip is rinsed with the silicate solutionpreferably by dipping the strip into the solution for at least 20seconds. After dipping, the strip is rinsed and dried. The silicatecoating provides excellent corrosion protection for the steel strip.

The silicate coated strip then is coated with a layer having a uniformthickness of the blended powder mixture of the thermosetting powder andorganosilane powder. The organic powder may be applied to the strip byelectrostatic spray guns such as disclosed in U.S. Pat. No. 5,059,446;incorporated herein by reference, or by passing the strip through anelectrostatically charged fluidized bed. If the fluidized bed is notelectrostatically charged, the strip must be preheated. The strip isheated to at least the curing temperature of the thermosetting resin byfirst passing the powder coated strip preferably through an inductioncoil. The heated strip then passes through another heater such asinfrared for maintaining the molten resin for sufficient time so thatthe organosilane becomes dissolved into and diffuses to a surface of thesteel strip. An important feature of the invention is that theorganosilane be soluble with the thermosetting resin. That is, theorganosilane must be capable of being melted as well as go into solutionwith the molten resin. During this time period, the organosilane alsomigrates toward the inorganic inner layer, e.g., silicate, therebyestablishing a tight covalent bond between the inorganic layer and thecrosslinked outer paint layer.

Details of a blended resin/organosilane mixture of the invention will bebetter understood from the following example. A powder containing athermosetting resin and optionally one or more fillers or pigments isprepared in a conventional manner. Thereafter, a solid, non-hydrolyzedlow molecular weight organosilane is blended with the powder mixture bycryogenic grinding until a homogeneous powder resin/organosilane paintcontaining at least 0.01 wt. % silane is formed.

A preferred embodiment of a metal substrate coated with an adherentcorrosion resistant composite coating of the invention formed from theabove described blended resin/organosilane powder mixture now will bedescribed. A steel strip is solvent cleaned of dirt, oil and othersurface contaminants. The strip then is rinsed for 20-30 seconds with asolution having a pH of about 12, a temperature of about 55°-60° C. andcontaining 0.005M of one of waterglass, sodium silicate or sodiummetasilicate and 0.005M of one of Ba(NO₃)₂ or Ca(NO₃)₂. After beingdried, the silica or silicate coated strip then is coated with the abovedescribed blended resin/organosilane mixture of the invention such aswith electrostatic spray guns or by being passed through a fluidizedbed. When being coated using electrostatic guns, the strip preferably isat ambient temperature. When being coated using a fluidized bed, thestrip preferably is preheated to an elevated temperature. The powdercoated strip then would be heated in a conventional manner such as byinduction, infrared and the like, to at least the curing temperature ofthe thermosetting resin. The strip would be maintained at thistemperature for sufficient time to melt the resin and silane, dissolvethe silane into the resin and diffuse the silane to the surface of theinorganic silicon coating. The strip now would be coated with anadherent, durable, corrosion resistant composite coating having an outercrosslinked paint layer bonded to an inner inorganic layer by anorganosilane. This coated steel would have particular advantages overordinary painted steels because the interface between the steel and thepaint, i.e., the silicon containing inorganic layer, is very resistantto moisture. Thus, the composite coated strip of the invention canadvantageously be used in wet environments.

It will be understood various modifications can be made to the inventionwithout departing from the spirit and scope of it. Therefore, the limitsof the invention should be determined from the appended claims.

What is claimed is:
 1. A metal having a corrosion resistant coating,comprising:a crosslinked layer coupled to a metal substrate, saidcrosslinked layer being the reaction product formed by curing a powder,said powder being a blended mixture of a thermosetting resin and asolid, non-hydrolyzed organosilane from the group consisting ofmonomeric, dimeric or oligomeric molecules, said organosilane includingone organofunctional group and one silicon containing group consistingof trialkoxysilyl or trichlorosilane, said organosilane being a solid atambient temperature and having a melting point no greater than thecuring temperature of said resin, said crosslinked layer being coupledto said substrate by said organosilane.
 2. The metal of claim 1 whereinthe concentration of said organosilane in said mixture is 0.01-10 wt. %.3. The metal of claim 1 wherein the coating includes a non-toxic,inorganic inner layer, said crosslinked layer being coupled to saidinner layer by said organosilane.
 4. The metal of claim 3 wherein saidinner layer is a silicate.
 5. The metal of claim 1 wherein saidcrosslinked layer has a thickness of at least 10 μm.
 6. The metal ofclaim 1 wherein said resin has a curing temperature of 100°-250° C.